Phloem Loading
Vascular plants produce nutrients such as sucrose in their leaves. These nutrients must them be transported to the rest of the shoot or to the root tips, where growth occurs.
The leaves are referred to as the source, and the shoot and root tips aew referred to as the sink.
A source is am organ that produces more sugars than it requires and a sink is an organ that consumes sugars for its won growth and storage.
There are two kinds of vascular conducting tissues in plants:xylem, which conducts carbohydrates, mainly sucrose.
The transport of sugars throughout the plant, called translocation, is carried out dy phloem cells.
Phloem cells are connected to one another, end-to-end, by sieve plates. there are perforated structuctures that create a direct connection between the cytoplasm of connected phloem cells.
phloem loading is the process whereby carbohydrates enter the sieve tubes at the sourcce. As a result of phloem loading, a high concentration of sugar develops in phloem cells near the source.
phloem loading results in a lowered eater potential, compared with adjacent xylem cells, causing water to move from xylem to phloem by osmosis.
This imflux of water creates a high turgor pressure near the source and lower turgor pressure near the sink, causing the movement of water and sugar from the source to the sink.
Sucrose is removed at the sink by various active transport mechanisms, depending on the cell type.
In some plant cells, a transport protein located in the membrame of a storage vacuole called a tonoplast, transports sucrose from the sink cell cytoplasm into the vacuole at the expense of ATP.
This reduces the concentration of sucrose in the sink cells, which emhances the move ment of sucrose from the phloem cells through the companion cells and into the sink cell.
All these events produce a continuous flow of water from xylem to phloem and then back to xylem.
water leaving phloem in the root moves upward primarily due to transpiration, not due to water potential differences between xylem and phloem.